Process of preparing iron and aluminum surfaces to receive organic coatings and solution therefor



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Patented July 25, I950 PROCESS OF PREPARING IRON AND ALU- MINUM SURFACES TO RECEIVE ORGANIC COATINGS AND SOLUTION THEREFOR Alfred Douty, Elkins Park, and Eugene Snyder, assignors to American Chemical Paint Company, Ambler, Pa., a corpo- Philadelphia, Pa.,

ration of Delaware No Drawing. Application April 19, 1944, Serial No. 531,778

8 Claims. 1

This invention relates to the art of preparing metal surfaces to receive surface protective coatings such as paint or the like. It is primarily concerned with the treatment of surfaces containing iron or aluminu x or their alloys, and is specifically cofcernd with the provision of a final rinsing solution to be applied to such surfaces before the final coating operation begins.

It has long been known that the adhesion and durability in service of surface protective coatings on articles of steel and aluminum are profoundly affected by the condition of the metal surface just prior to the application of the protective coating. It has been known that the quality of the protective coating under exposure to corrosive conditions, such as salt spray or high humidity atmosphere, is also affected by the condition of the surface prior to painting. For this reason it has become common industrial practice to clean the surfaces of iron and aluminum articles with some form of cleaning agent sufli ciently powerful to remove most of the superficial impurities.

We have discovered that commercial cleaning agents employed for thispurpose are likely to leave a residue on the surface which has a harmful effect on the adhesion and durability of the paint. This is true, even where surfaces which have been subjected to chemical cleaning of the type described are subsequently given a water rinse, because even vigorous treatment with a water spray is not sufiicient to remove all traces of th cleaning compounds with entrained deleterious matter, and because the water itself is seldom sufliciently free of soluble salts. We have developed a final rinsing treatment which overcomes the disadvantages incident to the use of titles, an acid component operative to render the surface relatively inert to further chemical change. The acids useful for this purpose are chromic acid, and the acids of phosphorus, or a mixture of both types. The proportion used is sucdasjo produce an extremelyhilutesolfition" ...h%.Y1l1i H value of 5. or? less, and preferably between pH 3 and pH 5. To secure uniform wetting action and quick run-off, without at the same time importing materials which might leave active or ionizable salts behind, a llOll-iOnlQ surface-acti ve agent capabl of reduci'fig surface thrones added tothe solution in amounts sufficient to reduce the surface tension to a value of the order of 50 dynes/cm. or less.

It is desirable to heat the final rinse solution at the time Of use to a temperaturengea; to the b o iling point, and to allow the work to remain in thefilufion until it attains the temperature of the bath. This seems to increase the efiiciency of the solution, and it also promotes rapid drying of the work after removal from the bath.

Protective coatings applied to the surfaces which have been treated as outlined are more firmly adherent and do not deteriorate as rapidly in service as similar coats applied without the use of this treatment. Under these circumstances, a detailed discussion of the theoretical aspects of the process seems to be unnecessary. However, it may be helpful to discuss briefly some of the conclusions tentatively held. Apparently the treatment is operative to reduce the reactivity of the metal surface, and to render relatively inert both the surface and any superficial blemishes or contaminants remaining upon it. With phosphoric acid, this effect is probably due to an actual interaction between the acid and the metal, producing an inert condition. With chromic acid the effect of the rinse is not so apparent, but it seems to be to passivate the surface, without producing any very marked interaction between acid and metal. In either case the rinse will render innocuous any traces of rust-stimulative anions such as chlorides, sulphates, or the like, or cations, such as sodium, calcium, or magnesium, which may happen to remain on the surface of the metal. 7

The presence of the surface-active component ensures that the solution will. quickly and evenly wet the surface, thereby also promoting uniform action by the acid component. Another purpose of the wetting agent is to promote the physical action of the solution by facilitating the run-off, so that it will entrain and remove extraneous matter as it drains away, and will leave only a thin film behind it. Thus the physical action of rinsing becomes more efiective, and at the same time less time is required to dry the work. Consequently there is less likelihood that the metal will be reoxidized while still wet. Furthermore, so thin a film of the rinsing solution remains on the surface that the amount of coloration due to traces of acid remaining after the surface has been dried is reduced to a Still further, traces of non-ionic wetting agents remaining after drying of the film do not in any way impair the quality or function of protective coatings applied. thereafter. This is in contrast to the effect produced where treating solutions are used which contain anionic agents, such as sulphated or sulphonated materials, or cationic agents, such as fatty acid amines and the like. Even small traces of the ionic type agents produce definitely undesirable efiects so far as the protectivc coating is concerned.

It is important both for reasons of effectiveness and for reasons of economy of operation to keep the final rinsing bath as free as possible of soluble salts of all kinds, especially chlorides. For this purpose it is preferable to apply a hot water rinse to the. metal immediately after the main cleaning treatment and just before the application of the final rinsing solution of this invention.

It will, of course, be very obvious that a rinsing solution of the type here proposed should contain very minute quantities of acid. It is sufficient to add from .0l% to .1% of either acid, adjusting the precise amount to attain the pH value most effective in relation to other factors, such as the nature of the cleaning agent previously used. Of the acids of phosphorus, we prefer to employ orthophosphoric, and to use the 75% grade.

The wetting agent employed plays a very important part in the solution. It is essential that it should not ionize in the solution to any appreciable degree. Otherwise it might itself be responsible for introducing and depositing undesirable compounds upon the surface.

Various llQfl-ionic emulsifying and wetting agentsare available. Some of these are found in nature, among these being the saponins, soap bark extract, the alginates, pectin, lecithin, casein, gum arabic, gum tragacanth, albumin, glue, starch, gelatin and agar-agar. These natural agents are, however, noticeably less effective than certain synthetic non-ionic surface-active agents now commercially available in many different forms.

The synthetic surface-tension depressants usein] in this process are of the general class of the substituted polyethylene glycols. Especially useful are those of the formula in which R stands for an alkyl, aryl, aralkyl, alkyl-aryl, or acyl radical containing at least 10 carbon atoms; n is an integer greater than 2; R1 stands for an alkyl, aryl, aralkyl, alkyl-aryl, or acyl radical or a hydrogen atom. Among the commercial products falling within this class may be mentioned Triton NE. This is a polyglycol in which n indicates an average length of between 8 and 12. Many such products may be found in the catalogs of the chemical supply industry, and

the names and properties of quite a few of these will be found in the March 1943 issue of Chemical Industries" in an article by Henry A. Goldsmith, appearing on pages 326 to 3:28.

It should be noted that, although the synthetic wetting agents are greatly to be preferred, as compared to non-ionic wetting and emulsifying agents found in nature, it is quite possible to ob-' tain good results with agents of the latter type if the quantity employed is sufiicient. By way of illustration, in a rinsing solution containing, say, .03% of phosphoric acid, an equivalent effect will be produced by the employment of 2% of saponin or, alternatively, about .025% of a synthetic non-ionic surface-active agent of the type herein discussed. The substituted polyglycol ethers are partiglarly csirablemategials be- TlEftlfiFarefii'dre stablginacidfsblution and alsobecause "they'iiow' more 'easilyff'ro'iil'thesurface" attl'idconolusion'ofjhefr: treatment. *ltwillyof cours'fbe' apparent: that, ofthe many types of surface-active agents known, only those which produce a lowering of the surface tension are to' be considered. Many non-ionic materials having excellent wetting and emulsifying properties do not lower the surface tension sufficiently, others even increase it.

The final rinse may be prepared on the spot, or a concentrate may be prepared in advance which is added to the proper amount of water to produce the concentration indicated.

Merely for the purposes of illustration three formulae are given below for the preparation of a final rinse solution of the type here involved.

FORMULA NO. 1

Rinsing solution 75% phosphoric acid gals. 0.2

Saponin lbs. 8.3 Tap water to make gals.

It will be noticed that, on a weight basis, the ratio of phosphoric acid to saponin is approximately 1:3, and the ratio of acid and saponin toether to the amount of water is less than 1%. This solution is particularly desirable for high speed commercial operation, because of the ease with which the pH value of the solution can be adjusted.

- FORMULA NO. 2

Rinsing solution l Chromic acid (CrOa) lbs. .50 Triton NE gals. .01 Taper water to make 100 gals.

It will be seen that, on a weight basis, the ratio of acid to wetting agent has been more than reversed, as compared to Formula No. 1. This reflects the notably greater efiectiveness of the synthetic wetting agents of the non-ionic type, which is well illustrated by the difierence in the amount required of the natural material, saponin, used in Formula No. 1, as compared to the synthetic polyglycol ether known as Triton produce no other harmful effects, they will occasionally show through a light finish coat.

FORMULA NO. 3

Rinsing solution 5 Chromic acid (CrOg) lbs. .25 75% Phosphoric acid gals. .02 Triton NE gals. .02

Tap water to make 100 gals.

This formula produces a double effect, since the phosphoric acid is particularly useful in removing superficial contaminants and also, being a freely soluble liquid, can be added as needed to bring the bath to the pH value indicated; and the chromic acid does not produce any of the so-called "rust blush" which. i sometimes produced by phosphoric acid. Furthermore, the presence of the chromic acid seems to reduce the tendency of the phosphoric acid to produce rust- 2 blush. The proportion between chromic acid and phosphoric acid in this formula may be varied as desired, so as to take full advantage of the respective properties of both acids.

For commercial purposes it is desirable that a concentrated solution should be prepared, which can be diluted with water at the time of use to produce the pH desired and which will at the pH indicated have a surface tension within the range specified. A concentrated solution of this sort, however, should not contain chromic acid, since this, in concentrated form, will react with the wetting agent to impair the effectiveness of both acid andagent. Accordingly, the following eX- ample is given to show the proportions of a concentrated material using phosphoric acid:

FORMULA N0. 4

Phosphoric acid concentrate Gallons 4 75% phosphoric acid .70 Igepal CTA .30

This material can be prepared and shipped without fear of deterioration and, if added to a water 4:. bath containing, say, about 3000 gallons, will produce a solution having the characteristics desired, namely a pH not greater than 5 and a surface tension of less than d./cm.

It should be emphasized, in connection with 50 the foregoing application as a whole, that, in the first place, the amount of acid used should be very slight, and'that the solution employed should have a pH value not substantially greater than 5 and preferably in the range from pH 3 to pH 5. In the second place, it should be reiterated that only those surface-active agents which are free of anionic or cationic functions should be used, and that of these agents, only those which are capable of lowering the surface tension of a dilute acid solution of the type herein described should be employed. In particular, it is noted that, although most natural wetting agents of the types recited herein will produce a reduction in surface tension to a value at least a low as 50 e5 d./cm. in such solutions at concentrations not substantially exceeding 2%, it is none the less true that there are other naturally occurring wetting agents in this general class which do not have this capability. Since the properties of these naturally occurring agents in this respect are well known, those experienced in the art will have no difliculty in selecting agents capable of meeting the requirements specified.

Although the addition of a substantial proportion of the wetting agent is not particularly harmful, it is wasteful, and should be avoided. After saponin, for example, has been added to the extent of about 2%, there seems to be practically no decrease in the surface tension of the solution upon the addition of further amounts of this agent. As noted above, considerably smaller proportions of the synthetic non-ionic wetting agents will be found to be sufficient. The addition of fractional percentages of such agents produces a very striking effect. The addition of further increments is accompanied by a progressively declining rate in the reduction of surface tension up until concentrations in the neighborhood of 2% are reached, beyond which the synthetic agents also seem to lose the capacity to produce much further change in the surface tension of the solution.

We claim:

1. A dilute aqueous rinsing solution to be applied to ferriferousand aluminiferous metal surfaces after a primary cleaning treatment and before the application of a surface-protective coating, said solution consisting essentially of water and from .01% to 10% of an acid component of the class which consists of chromic acid, phosphoric acid, and mixtures thereof; and from .001% to about 2.00% of a surface-active component consisting of a non-ionic wetting agent operative to reduce the surface-tension of the solution to a value of 50 dynes/cm. or less.

2. The dilute rinsing solution of claim 1 in which the non-ionic wetting agent is a slu'facetension depressant having the generic formula R0 (C2H4O) 11131 in which R stands for a member of the group which consists of the alkyl, aryl, aralkyl, alkylaryl, and acyl radicals, containing at least 10 carbon atoms; n is an integer greater than 2; and R1 stands for a member of the group which consists of alkyl, aryl, aralkyl, alkyl-aryl, and acyl radicals and hydrogen.

3. A dilute rinsing solution according to claim 1 and having a pH value greater than 3 and less than 5.

4. In a process for protecting ferriferous and aluminiferous surfaces wherein a preliminary step is subjecting the surface to a cleaning treatment and the final step is the application of an organic surface-protective coating, the intermediate step which is the final chemical treatment prior to the application of said organic coating,

and which comprises a final rinsing of the surface with a dilute aqueous solution having a surface tension lower than 50 dynes/cm., and having a pH value not exceeding 5, which solution consists essentially of water and from .0l% to .10% of an acid component of the class which consists of chromic acid, phosphoric acid, and mixtures thereof; and a surface-active component consisting of a non-ionic wetting agent operative to reduce the surface tension of the solution.

5. The intermediate step of a process as defined in claim 4 which is characterized in the use of a rinsing solution in which the non-ionic wetting agent is a surface-tension depressant selected from the class of substituted polyethylene glycols and having the generic formula RO (C2H40) nRi in which R stands for a member of the group which consists of the alkyl, aryl, aralkyl, alkylaryl, and acyl radicals, containing at least 10 carbon atoms: 11 is an integer greater than 2; and R1 stands for a member of the group which consists of the alkyl, aryl, aralkyl, alkyl-aryl, or acyl radicals and hydrogen.

6. The process of preparing ferriferous and aluminiferous surfaces to receive organic protective coatings which process includes the steps of: subjecting the surfaces, after conventional chemical cleaning treatment; to a final rinsing treat- .ment with the dilute solution of claim 1, ata

temperature near to the boiling point of the solution; allowing the solution to drain from the sur faces and air-drying the surfaces.

7. The process of preparing ferriferous and aluminiferous surfaces to receive organic protective coatings which process includes the steps of subjecting the surfaces, after conventional chemical cleaning treatment to rinsing with water;

subjecting the rinsed surfaces to a final rinsing treatment with the dilute solution of claim 1, at a temperature near to the boiling point of the solution; allowing the solution to drain from the surfaces and air-drying the surfaces.

8. The process of preparing ferriferous andl aluminiferous surfaces to receive organic protective coatings which process includes the steps of: subjecting the surfaces, after conventional chemical cleaning treatment, to a final rinsing I the solution drains away.

treatment with the dilute solution of claim 1, at a temperature near to the boiling point of the solution and for a time sufl'icient to effect a substantial increase in the temperature of the articles; and thereafter air-drying at temperatures materially above room temperature, as and after ALFRED DOUTY. EUGENE SNYDER.

REFERENCES CITED I The following references are of record in th file of this patent:

OTHER REFERENCES [mos NE-Bulltin of 'Rohm and Haas 00., Phila, Dec. 1943, pages 12 and 14. 

4. IN A PROCESS FOR PROTECTING FERRIFEROUS AND ALUMINIFEROUS SURFACES WHEREIN A PRELIMINARY STEP IS SUBJECTING THE SURFACE TO A CLEANING TREATMENT AND THE FINAL STEP IS THE APPLICATION OF AN ORGANIC SURFACE-PROTECTIVE COATING, THE INERMEDIATE STEP WHICH IS THE FINAL CHEMICAL TREATMENT PRIOR TO THE APPLICATION OF SAID ORGANIC COATING, AND WHICH COMPRISES A FINAL RINSING OF THE SURFACE WITH A DILUTE AQUEOUS SOLUTION HAVING A SURFACE TENSION LOWER THAN 50 DYNES/CM., AND HAVING A PH VALUE NOT EXCEEDING 5, WHICH SOLUTION CONSISTS ESSENTIALLY OF WATER AND FROM .01% TO .10% OF AN ACID COMPONENT OF THE CLASS WHICH CONSISTS OF CHROMIC ACID, PHOSPHORIC ACID, AND MIXTURES THEREOF; AND A SURFACE-ACTIVE COMPONENT CONSISTING OF A NON-IONIC WETTING AGENT OPERATIVE TO REDUCE THE SURFACE TENSION OF THE SOLUTION. 